Synthesis of a WO₃ photocatalyst with high photocatalytic activity and stability using synergetic internal Fe³⁺ doping and superficial Pt loading for ethylene degradation under visible-light irradiation

Abstract

Ethylene (C₂H₄) is harmful for storage and fresh-keeping of fruits and vegetables. Photocatalytic technology is an effective and environmentally friendly approach for degrading ethylene. Herein, we first report the synthesis of a WO₃ photocatalyst with high photocatalytic activity and stability using synergetic internal Fe³⁺ doping and superficial Pt loading for ethylene degradation under visible-light irradiation. Internal Fe³⁺ doping not only enhances the visible-light absorption but also improves the stability to some degree due to more positive reduction potential of Fe³⁺/Fe²⁺ than that of W⁶⁺/W⁵⁺ and separation efficiency of the photo-generated carriers. Furthermore, the loading of Pt as a co-catalyst through the photo-reduction of H₂PtCl₆ on the surface of Fe-doped WO₃ promotes the effective transfer of photo-generated electrons and reduces the photo-corrosion of WO₃. Due to the synergistic effect, extremely high degradation speed can be achieved by doping 0.25 mol% Fe and loading 0.75 wt% Pt for WO₃, which exceeds that of pristine WO₃ by about 3.3 times under visible-light irradiation. In addition, due to the excellent stability of Pt@Fe-doped WO₃, it has good industrial application prospects in the field of photocatalysis.

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1. Introduction

Ethylene (C₂H₄) as a plant hormone has numerous effects on the growth, development and storage of fruits and vegetables.¹ During the process of growth, ethylene plays a useful role in promoting the ripening of fruits and vegetables. However, after harvesting of fruits and vegetables, ethylene can be constantly produced through respiration, which can accelerate the senescence of fruits and vegetables and is harmful for their storage and fresh-keeping.² Therefore, it is important to remove ethylene in the storage environment. Photocatalytic technology is considered as an effective and environmentally friendly approach to degrade ethylene molecules into CO₂ and H₂O through photocatalytic oxidation.³ However, due to the nonpolar nature and high bond energy of ethylene molecules, they cannot be degraded easily.⁴ Thus, it is a great challenge to find and design efficient photocatalysts for use in photocatalytic ethylene degradation.

With respect to the photocatalytic degradation of ethylene, titanium dioxide has been proven to be the most important photocatalyst due to its high activity, long-term stability, low toxicity and low cost.^5,6 However, it can only respond to the ultraviolet light, which only accounts for 5% of the solar spectrum, due to its intrinsic wide band gap of about 3.2 eV.⁷ To make good use of sunlight, semiconductor photocatalysts should have good visible-light absorption. WO₃ has a smaller band gap (2.5–2.8 eV) than TiO₂ (3.2 eV) and has many inherent advantages such as low cost, nontoxicity and high oxidation power of valence band holes.^8,9 In recent years, WO₃ has been studied as a photocatalyst in many applications such as the degradation of organic dyes, water splitting, and reduction of CO₂.^10–17 Due to the requirement of high selectivity of photocatalysts for ethylene degradation, only some specific photocatalysts can be used to degrade ethylene. As far as we know, there has been no report on the photocatalytic ethylene degradation using WO₃ as the photocatalyst. Although the use of WO₃ has many advantages, there are still some drawbacks, which seriously restrict its performance. The first is the limited visible-light absorption; to solve this problem, doping metals is considered as a good strategy, and these metals include Ti, Cu, Mo and Fe.^18–21 The second is that WO₃ is easily photo-corroded due to self-reduction of W⁶⁺, which is disadvantageous for photocatalytic activity.

In this study, we first report the synthesis of a WO₃ photocatalyst with high photocatalytic activity and stability using synergetic internal Fe³⁺ doping and superficial Pt loading for ethylene degradation under visible-light irradiation. The optimal calcination temperature for WO_2.72 to produce WO₃, the optimal doping amount of Fe³⁺ and loading amount of Pt are studied in our experiment systematically. Internal Fe³⁺ doping not only enhances the visible-light absorption but also promotes the stability to some degree due to more positive reduction potential of Fe³⁺/Fe²⁺ than that of W⁶⁺/W⁵⁺. During the photocatalytic reaction, the photo-generated electrons are more easily captured by Fe³⁺ and are converted to Fe²⁺, which can avoid the photo-corrosion of WO₃ effectively and promote the separation of photo-generated carriers. Furthermore, the loading of Pt as a co-catalyst through the photo-reduction of H₂PtCl₆ on the surface of Fe-doped WO₃ promotes the effective transfer of photo-generated electrons and reduces the photo-corrosion of WO₃. The synergistic effect of internal Fe³⁺ doping and superficial Pt loading can realize the best photocatalytic activity for the degradation of ethylene under visible-light irradiation. The synergetic mechanism can be expressed clearly using a metaphor: the photo-generated electrons are water, the internal doping of Fe³⁺ can be represented by a water reservoir and superficial Pt co-catalyst functions as a water pump. Thus, the photo-generated electrons and holes can be separated efficiently. After a certain time, the water reservoir should be released. Calcination is an effective way to convert Fe²⁺ back to Fe³⁺; thus, the photocatalyst can realize excellent recyclability. The effective synergistic effect can provide a good model for other photocatalytic systems.

2. Experimental

2.1 Material synthesis

2.1.1 Synthesis of WO_2.72 and WO₃. WO_2.72 was synthesized according to a previously reported method.¹⁸ Briefly, 2 g of WCl₆ was added into 80 ml of absolute ethanol, and a clear and bright yellow solution was formed with vigorous stirring. The solution was then transferred into a 100 mL-Teflon-lined autoclave and heated at 160 °C for 24 h. When cooled to room temperature, the blue product was filtered, washed with ethanol and distilled water several times and then dried in air at 60 °C overnight. In order to determine the optimal calcination temperature for the synthesis of WO₃, the blue WO_2.72 powder was calcined at 300, 350, 400, 450, and 500 °C for 1.5 hours in air with a heating rate of 10 °C min⁻¹.

2.1.2 Synthesis of Fe-doped WO₃. The Fe-doped WO₃ was synthesized by a similar method except that a different amount of iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) was added. The theoretical contents of Fe in WO₃ were 0.1, 0.25, 0.5, 0.75 and 1.0 mol%. Then, the as-prepared samples were calcined at 450 °C in air for 1.5 hours to obtain the Fe-doped WO₃.

2.1.3 Synthesis of Pt@Fe–WO₃. Pt@Fe–WO₃ was synthesized via the photo-reduction method. First, 0.5 g of 0.25 mol% Fe-doped WO_2.72 was dispersed in 100 mL distilled water by sonication for 5 min. Then, a certain volume of H₂PtCl₆ (0.0772 M) was added to the above suspension. After stirring for 30 minutes, 1 mL methanol was added. The system was irradiated by full spectrum light for 30 minutes. The products were filtrated and washed with water. After drying at 60 °C, the products were calcined at 450 °C in air for 1.5 hours.

2.2 Material characterization

XRD was characterized on a Bruker AXS D8 diffractometer equipped with Cu Kα excitation. UV-vis DRS of the products was conducted using a Shimadzu UV 2550 spectrophotometer equipped with an integrating sphere. Morphology was observed by a field emission scanning electron microscope (SEM) (Hitachi S-4800) equipped with an energy dispersive spectrometer (EDS). XPS measurements were obtained using a Thermo Fisher Scientific Escalab 250 spectrometer, and the peak positions of various elements were calibrated by C 1s (284.8 eV). The PL curves using 350 nm as the excitation wavelength were surveyed by a Hitachi F-4500 fluorescence spectrophotometer. Photoelectrochemical measurements with a three-electrode system were conducted using a Princeton Applied Research EG&G 263A electrochemical workstation. The platinum sheet served as the counter electrode; Ag/AgCl served as the reference electrode, and catalyst-covered FTO glass served as the working electrode. The electrolyte solution was 0.5 M Na₂SO₄. A 300 W Xe-arc lamp equipped with an AM 1.5 cut-off filter was used as the light source.

2.3 Photocatalytic test

The photocatalytic ethylene degradation experiment was carried out in a special reactor with a volume of 320 mL; 0.4 g of photocatalyst was sprinkled uniformly at the bottom of the reactor, and the reactor was sealed by a silicone rubber stopper. Then, 0.4 mL of high-purity ethylene gas was injected into the reactor, and the reaction temperature of the system was maintained at 15 °C with cooling water. The initial concentration of ethylene (C₂H₄) was 1250 ppm. A 300 W Xe lamp with a 420 nm cut-off filter was used to provide the visible light. Prior to irradiation, the system was magnetically stirred in the dark for 2 h to establish an adsorption–desorption equilibrium of the gas with the samples. When a balance was reached, the photocatalytic test began. First, 60 μL of the gas mixture was sampled from the reaction system at a certain time interval and was analyzed by an online gas chromatograph (Shimadzu GC-2014C) equipped with a thermal conductivity detector (TCD). A blank test was also conducted without photocatalyst to prove that the degradation of ethylene was entirely caused by the photocatalytic reaction. The degradation percentage is expressed as C/C₀, where C₀ is the initial concentration of ethylene and C represents the corresponding concentration at a certain sampling time. The stability was tested as follows: after each ethylene oxidation reaction, the photocatalyst was collected and calcined at 450 °C for 0.5 hours to remove possible residuary organics. Then, the regenerated product was employed to conduct another round of ethylene degradation test under the same conditions.

3. Results and discussion

3.1 X-ray diffraction study of the samples

The crystallographic structures of the samples were characterized by X-ray diffraction (XRD). The XRD spectra of untreated WO_2.72 and WO_2.72 calcined at different temperatures are shown in Fig. 1. As can be seen, the crystallinity of all products improved after calcination. The diffraction peaks gradually transformed from WO_2.72 (JCPDS no. 36-101) to WO₃ (JCPDS no. 43-1035) with increase in calcination temperature. When the calcination temperature reached 400 °C, the sample entirely transformed into WO₃. Therefore, the calcination temperature is a very important factor for the formation of WO₃ from WO_2.72.

Fig. 1 XRD patterns of untreated WO_2.72 and WO_2.72 calcined at different temperatures.

As shown in Fig. 2a, after calcination at 450 °C, all the diffraction peaks of the as-prepared samples are nearly similar to that of pure WO₃. With the increase in Fe content, no apparent diffraction peaks of iron oxides or other impurity phases can be detected, illustrating that Fe³⁺ ions were doped into the crystal lattice of WO₃.²¹ However, from the enlarged peaks in Fig. 2b, we infer that the peaks in the Fe-doped products display apparent shifts compared to that of original WO₃. With the increase in Fe content, the diffraction peaks gradually shifted to a small angle. Because the radius of Fe³⁺ (0.64 Å) is larger than the radius of W⁶⁺ (0.62 Å), slight lattice distortion of WO₃ was observed. According to the Bragg's law, the diffraction angle will become smaller due to larger lattice spacing, which can explain the result of the peak offset. This phenomenon further indicates that the Fe³⁺ ions were doped into the crystal lattice of WO₃ successfully. Moreover, the XRD peaks of 0.75 wt% Pt@0.25 mol% Fe–WO₃ were similar to those of 0.25 mol% Fe–WO₃, and no clear peaks of Pt were detected, which might be caused by the low amount and high dispersion of Pt nanoparticles.

Fig. 2 The XRD spectra (a) and the enlarged XRD spectra (b) of the pure WO₃, Fe-doped WO₃ and Pt@Fe–WO₃ samples.

3.2 UV-vis spectroscopy

The UV-vis diffuse reflectance spectra of bare WO₃, Fe-doped WO₃ and Pt@Fe–WO₃ samples are shown in Fig. 3. As can be seen, bare WO₃ displays a sharp absorption edge at around 470 nm and a corresponding band gap energy of about 2.64 eV, which is consistent with previous results (2.5–2.8 eV).⁸ All products exhibited absorption in both UV and visible-light regions. Due to the low doping amount of Fe³⁺, the absorption edge only has a small red shift. Based on the enlarged view inserted in Fig. 3, we observe that the red shift of the absorption increases gradually with the increase in Fe³⁺ doping from 0.1 to 1.0%. The enhanced visible-light absorption of Fe-doped WO₃ is beneficial for the photocatalytic reaction, and it also confirms that Fe³⁺ was indeed doped into the lattice of WO₃. After photo-reduction of H₂PtCl₆, the light absorption increased in the 500–800 nm region, which was ascribed to the loading of Pt nanoparticles on the surface.

Fig. 3 UV-vis diffuse absorption spectra of the as-prepared samples.

3.3 Morphology and structure of nanostructures

The morphologies of the obtained samples were studied by SEM (Fig. 4). It can be observed that the WO_2.72 product possesses uniform urchin-like hierarchical structures with sizes ranging from 600 to 800 nm (Fig. 4a), which is consistent with the results from a previous report.²² The morphology of the as-synthesized urchin-like WO_2.72 architectures calcined at 450 °C is shown in Fig. 4b. After calcination, the original urchin-like hierarchical structures transformed into hierarchical microspheres consisting of nanoflakes due to recrystallization. The detailed morphologies of the as-synthesized urchin-like WO_2.72 architectures calcined at different temperatures are shown in Fig. S1.† As the temperature increased, the transformation process started gradually and the thickness of the nanoflakes increased. Combined with the result of XRD, we infer that the urchin-like WO_2.72 has been completely converted to WO₃ at 400 °C. As shown in Fig. 4c, the morphology of 0.25 mol% Fe-doped WO₃ is also composed of nanosheet arrays but with slight disorder compared with the morphology of un-doped WO₃. In addition, it should be noted that 0.75 wt% Pt@0.25 mol% Fe–WO₃ exhibits no significant morphological change after 9 time consecutive photocatalytic degradation experiments (Fig. 4d), indicating that the loading of Pt by photo-reduction and calcination at 450 °C for 0.5 h after every cycle experiment have almost no influence on the morphology.

Fig. 4 SEM images of (a) bare WO_2.72, (b) pure WO₃ (WO_2.72 calcined at 450 °C for 1.5 h), (c) 0.25 mol% Fe-doped WO₃ (0.25 mol% Fe-doped WO_2.72 calcined at 450 °C for 1.5 h), (d) 0.75 wt% Pt@0.25 mol% Fe–WO₃ after 9 time photocatalytic degradation experiments.

Fig. 5 displays the EDS and mapping analysis of the 0.75 wt% Pt@0.25 mol% Fe–WO₃ sample. In Fig. 5a, W, O, Fe and Pt can be detected by the EDS measurements. To intuitively observe the elemental distribution, EDS mapping was carried out. The detailed results can be seen in Fig. 5b–f. These results further prove the successful Fe doping and Pt loading in the WO₃ system.

Fig. 5 (a) EDS, (b) SEM and (c–f) mapping analysis of 0.75 wt% Pt@0.25 mol% Fe–WO₃ sample.

3.4 X-ray photoelectron spectroscopy study

To assess the surface chemical status of Fe-doped WO₃ before and after irradiation, X-ray photoelectron spectroscopy (XPS) was carried out. Fig. 6a shows the full XPS spectra of bare WO₃ and Fe-doped WO₃ before and after irradiation; the corresponding peaks of O 1s, C 1s and W 4f can be observed clearly. The peak of C 1s located at 284.8 eV is used to calibrate the peak positions. Due to the small doping amount of Fe, the XPS peak of Fe³⁺ 2p cannot be easily seen in Fig. 6a. From Fig. 6b and c, we can see that the peaks of W 4f (35.28 eV for W 4f^7/2 and 37.4 eV for W 4f^5/2) and O 1s (530.3 eV) for Fe-doped WO₃ before irradiation exhibit a slightly negative shift relative to the peaks of W 4f (35.8 eV for W 4f^7/2 and 37.9 eV for W 4f^5/2) and O 1s (530.64 eV) for un-doped WO₃. From the high-resolution Fe³⁺ 2p spectrum (Fig. 6d), we observe that the two small peaks located at 715.35 eV and 724.86 eV, corresponding to Fe 2p^3/2 and Fe 2p^1/2, exhibit slightly positive shifts compared to that of Fe₂O₃ reported previously (710.7 eV for Fe 2p^3/2 and 724.3 eV for Fe 2p^1/2).²³ The shift of the peaks in the XPS spectrum is due to different electronegativities of W and Fe; the electronegativity of W is relatively smaller than that of Fe.^24,25 When Fe³⁺ is doped into WO₃, Fe³⁺ replaces a part of W⁶⁺ to form an Fe–O–W bond. Compared with the Fe–O–Fe bonding in Fe₂O₃, the bonding between Fe and O is slightly enhanced in the Fe–O–W bond, and the bonding between W and O is slightly reduced compared with that in W–O–W. Therefore, the peak of Fe 2p has a slightly positive shift, and the peaks of W 4f and O 1s have slightly negative shifts. After the photocatalytic reaction, the binding energy positions of Fe 2p exhibit slightly negative shifts (713.74 eV for Fe 2p^3/2 and 722.77 eV for Fe 2p^1/2); the binding energy positions of W 4f (35.62 eV for W 4f^7/2 and 37.78 eV for W 4f^5/2) and O 1s (530.6 eV) for Fe-doped WO₃ after irradiation exhibit a slightly positive shift relative to that of Fe-doped WO₃ before irradiation. The binding energy positions of W 4f and O 1s for Fe-doped WO₃ after the photocatalytic reaction are almost the same as those of un-doped WO₃. According to the variation in binding energy positions, we conclude that the reduction potential of Fe³⁺/Fe²⁺ is more positive than that of W⁶⁺/W⁵⁺; therefore, Fe³⁺ can more easily capture photo-generated electrons to generate Fe²⁺ during the photocatalytic reaction, which results in decrease in the electronegativity. Therefore, the bonding between O and Fe is weakened and the bonding between O and W is strengthened in Fe–O–W. Simultaneously, the presence of Fe³⁺ can prevent the photo-corrosion of WO₃ (W⁶⁺ changes into W⁵⁺ by photo-generated electrons), which is beneficial for the photocatalytic reaction.

Fig. 6 (a) XPS survey spectra and (b and c) high-resolution XPS spectra of W 4f and O 1s for pure WO₃ and 0.25 mol% Fe-doped WO₃ before and after irradiation, (d) high-resolution XPS spectra of Fe 2p for 0.25 mol% Fe-doped WO₃ before and after irradiation.

3.5 Photocatalytic activity

The photocatalytic activities of WO₃, Fe-doped WO₃ and Pt@Fe-doped WO₃ were evaluated by performing photocatalytic ethylene degradation under visible-light irradiation. To obtain the optimal calcination temperature for WO_2.72 to determine the optimal photocatalytic performance for initial WO₃, multi-group degradation experiments were performed under the irradiation of full spectrum light. The detailed results can be seen in the ESI† (Fig. S2a). Among the studied samples, the sample calcined at 450 °C exhibited optimal photocatalytic ethylene degradation performance; thus, we chose 450 °C as the optimal annealing temperature. In addition, the optimal doping amount of Fe³⁺ ions was also chosen in our experiment, and the detailed result can be seen in the ESI† (Fig. S2b). When the doping amount of Fe³⁺ ions reached 0.25 mol%, the sample exhibited optimal photocatalytic performance under irradiation with full spectrum light. More doping amount caused decrease in the photocatalytic activity due to introduction of too much recombination centers.^26,27 The photocatalytic activity can be further improved by loading of Pt as a co-catalyst, and the optimal loading amount of Pt is 0.75 wt%; the detailed result can be seen in ESI† (Fig. S2c). After all the optimal parameters were chosen, we performed the photocatalytic degradation of ethylene using no photocatalyst, pure WO₃, 0.25 mol% Fe–WO₃ and 0.75 wt% Pt@0.25 mol% Fe–WO₃ under visible-light irradiation. The detailed results are shown in Fig. 7a. Without a photocatalyst, the concentration of ethylene was almost invariable, which proved that the degradation of ethylene is entirely caused by the photocatalytic reaction. After Fe³⁺ ions were doped into WO₃, the photocatalytic activity improved almost 2 times than that of pure WO₃. To further improve the photocatalytic activity, Pt as a co-catalyst was loaded on the 0.25 mol% Fe–WO₃ sample through photo-reduction. The 0.75 wt% Pt@0.25 mol% Fe–WO₃ sample exhibited the optimal photocatalytic activity and could degrade all of the ethylene in four hours under irradiation of visible light. From Fig. 7c, we can see that the concentration of CO₂ increased gradually during the process of photocatalytic degradation of C₂H₄, and the results illustrate that 80% C₂H₄ can be completely degraded to CO₂. The other partially oxidized intermediate species contain mainly CO and a small amount of formaldehyde. Stability is a very important criterion for the practical application of a photocatalyst. The recyclability of the photocatalyst was studied by performing consecutive degradation experiments under irradiation of visible light. After each cycle, the sample was collected and calcined at 450 °C for 0.5 hour to remove possible residuary organics. The result of the cycle experiment can be seen in Fig. 7b; the sample still maintained the initial photocatalytic activity after nine cycles, revealing that the photocatalyst has excellent recyclability. To prove the important effect of Fe doping, a contrast experiment was carried out using 0.75 wt% Pt–WO₃ and 0.75 wt% Pt@0.25 mol% Fe–WO₃ as photocatalysts under irradiation with full spectrum light. The detailed results can be seen in Fig. 7d. The results show that the photocatalytic activity of 0.75 wt% Pt@0.25 mol% Fe–WO₃ is superior to that of 0.75 wt% Pt–WO₃, thereby illustrating that Fe doping in the system plays an important role during the photocatalytic degradation. The photocatalytic degradation activity of 0.75 wt% Pt@0.25 mol% Fe–WO₃ sample was compared with that of other photocatalysts such as Pt–TiO₂, Ag–ZnO, AuAg–ZnO, BiVO₄/P25, and In₂O₃–Ag–Ag₃PO₄; the detailed information can be found in Table S1 (ESI†). Under visible-light irradiation, the photocatalytic C₂H₄ degradation activity of 0.75 wt% Pt@0.25 mol% Fe–WO₃ was better than those of BiVO₄/P25 and In₂O₃–Ag–Ag₃PO₄. Under full light irradiation, its activity was no less than those of Ag–ZnO and AuAg–ZnO, but it was poorer than that of Pt–TiO₂. This proves that Pt@Fe–doped WO₃ has a good application prospect.

Fig. 7 (a) Photocatalytic degradation of ethylene using no catalyst, pure WO₃, 0.25 mol% Fe–WO₃ and 0.75 wt% Pt@0.25 mol% Fe–WO₃ under visible-light irradiation; (b) the photocatalytic stability experiment of 0.75 wt% Pt@0.25 mol% Fe–WO₃ under visible-light irradiation; (c) photocatalytic process of C₂H₄ degradation and CO₂ generation for 0.75 wt% Pt@0.25 mol% Fe–WO₃ under visible-light irradiation and (d) the comparison of 0.75 wt% Pt@0.25 mol% Fe–WO₃ and 0.75 wt% Pt–WO₃ in photocatalytic ethylene degradation under full spectrum light irradiation.

3.6 Photoelectrochemical measurement and photoluminescence spectra

Photoelectrochemical tests and PL spectra are usually used to determine the separation ability of photo-excited carriers.^28–30Fig. 8a shows the results of the transient photocurrent response of pure WO₃ and 0.25 mol% Fe–WO₃ under AM 1.5 light irradiation in 0.5 M Na₂SO₄ aqueous solution at a bias potential of 1.23 V. Clearly, the 0.25 mol% Fe–WO₃ sample shows larger photocurrent density than pure WO₃ at the same bias potential, suggesting that the photo-generated electron–hole pairs in Fe-doped WO₃ can be easily separated compared to those in pure WO₃. Fig. 8b displays the PL intensity of pristine WO₃, 0.25 mol% Fe–WO₃ and 0.75 wt% Pt@0.25 mol% Fe–WO₃. As can be seen, the PL intensity of 0.25 mol% Fe–WO₃ is lower than that of pristine WO₃, demonstrating a lower electron–hole recombination rate for 0.25 mol% Fe–WO₃ compared with that of pristine WO₃. This result is consistent with the result of the transient photocurrent response. Furthermore, the PL intensity further decreased when 0.75 wt% Pt was loaded onto 0.25 mol% Fe–WO₃, indicating that the recombination rate of electrons and holes further decreased. These results are in good agreement with photocatalytic activity.

Fig. 8 (a) Photocurrent curves of pure WO₃ and 0.25 mol% Fe-doped WO₃; (b) PL spectra of pure WO₃, 0.25 mol% Fe-doped WO₃ and 0.75 wt% Pt@0.25 mol% Fe–WO₃.

3.7 Photocatalytic mechanism

From all the above results, the possible photocatalytic mechanism is shown in Fig. 9. When the photocatalyst is irradiated using visible light (λ ≥ 420 nm), WO₃ absorbs photons to produce photo-generated electrons and holes. In previous studies, it has been reported that Fe³⁺ usually acts as an electron acceptor during the photocatalytic O₂ production for WO₃.^31,32 Also, according to the XPS results, we can conclude that Fe³⁺ more easily captures photo-generated electrons to generate Fe²⁺ during the photocatalytic reaction due to more positive reduction potential of Fe³⁺/Fe²⁺ (0.771 V) than that of W⁶⁺/W⁵⁺.³³ This can avoid the photo-corrosion of WO₃ effectively and decrease the recombination of photo-generated carriers. Furthermore, Pt as a co-catalyst was loaded onto the surface of Fe-doped WO₃ through photo-reduction of H₂PtCl₆. The work function of WO₃ (5.7 eV) is larger than that of Pt (5.1 eV); when they are in contact with each other, the electrons migrate from Pt to the conduction band of WO₃ to achieve equilibration of the Fermi level.³⁴ The formation of the built-in electric field can promote the effective transfer of photo-generated electrons to Pt sites under visible irradiation, which is beneficial for the separation of photo-generated carriers and the reduction of the photo-corrosion of WO₃. Due to the low CB level of WO₃, the process of the single-electron reduction of oxygen cannot proceed. The accumulated electrons on the Pt particles are then used for the multi-electron reduction of oxygen molecules to form H₂O₂ and then for further production of hydroxyl radicals (˙OH); loading of Pt can accelerate the reaction of the photo-excited electrons with O₂.^35,36 The photo-generated holes with high oxidation potential react with the surface-absorbed H₂O to produce hydroxyl radicals. The hydroxyl radicals as extremely strong oxidants can degrade the C₂H₄ molecules effectively. The synergistic effect of internal Fe³⁺ doping and superficial Pt loading can be expressed by a metaphor: the photo-generated electrons are water, the internal doping of Fe³⁺ functions as a water reservoir, and the superficial Pt co-catalyst functions as a water pump. After a certain time, the water reservoir should be released. Calcination is an effective way to convert Fe²⁺ back to Fe³⁺, and the photocatalyst can realize excellent recyclability.

Fig. 9 Proposed schematic diagram of charge transfer in the Pt@Fe–WO₃ system under visible-light irradiation.

4. Conclusions

We first report the synthesis of WO₃ nanostructures with high photocatalytic activity and stability using synergetic internal Fe³⁺ doping and superficial Pt loading for ethylene degradation under visible-light irradiation. The optimal doping amount of Fe³⁺ is 0.25 mol% and the optimal loading amount of Pt is 0.75 wt%; the synergetic modification can result in about 3.3-fold improvement in the degradation rate than that of pure WO₃ in photocatalytic ethylene degradation. The doped Fe³⁺ and loaded Pt particles can capture photo-generated electrons to reduce the recombination of photo-generated carriers and protect WO₃ from photo-corrosion. This photocatalyst shows excellent photocatalytic degradation stability, revealing that it has an excellent application prospect.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by a research grant from the National Natural Science Foundation of China (No. 51602179, 21333006, 21573135 and 11374190), and the National Basic Research Program of China (973 Program, No. 2013CB632401), Recruitment Program for Young Professionals, China and Taishan Scholar Foundation of Shandong Province, China.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cy02375a

‡ These authors contributed equally to this work.

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